Intervertebral Implant

ABSTRACT

An intervertebral implant comprises first and second parts having outer surfaces engaging adjacent vertebral surfaces. An insert between these parts provides relative movement therebetween. The first and/or second parts may have engagement means in the form of apertures for engagement with insertion instruments. The first and second parts may nest together, in the absence of the insert, for insertion into the intervertebral space. The boundaries of the implant may form a working space which would include insertion instruments therein. A detent and detent recess may be provided for engagement of the insert and one of said first and second parts.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/185,781 filed Jul. 21, 2005, which is a continuation of applicationSer. No. 10/018,402, filed Jun. 12, 2002, now U.S. Pat. No. 6,936,071,which is a national stage entry of PCT/EP99/04628 filed Jul. 2, 1999,the entire disclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The invention relates to an intervertebral implant, having an upper partthat has a support face for a vertebra and a lower part that has asupport face for an adjacent vertebra, on each of which parts engagementelements, which are accessible from one side of the intervertebralimplant, for a manipulation instrument are disposed, in order tominimize the structural height of the intervertebral implant uponinsertion into an intervertebral space.

One such intervertebral implant is known for instance from U.S. Pat. No.5,314,477. This intervertebral implant is used to replace a disk removedfrom the intervertebral space, and accordingly the intervertebralimplant must have a relatively low structural height, since it has tofit into the gap between vertebrae. This is particularly difficult if anadditional pivot insert is also embedded between the upper part and thelower part, as is the case in the known intervertebral implant of U.S.Pat. No. 5,314,477.

But even in two-piece intervertebral implants, difficulties also arise,especially if the implants also have pins and other protrusions on theirsupport faces that are intended for anchoring the intervertebral implantin the bone. Often, these parts can be inserted only by widening theintervertebral space greatly. Not only is this difficult, but it alsopresents the risk of injuries.

Since the intervertebral space has a relatively low height, it is alsodifficult for engagement elements that a manipulation instrument canengage to be secured to both parts of the intervertebral implant. It isconventional to have such manipulation instruments engage the upper partand the lower part separately, for instance by means of pins that areinserted into bores on the upper part and lower part, so that with themanipulation instrument, the two parts of the intervertebral implant canbe inserted into the intervertebral space and can optionally also bevaried in terms of their spacing from one another, thereby allowing acertain spreading open of the intervertebral space. In this respect,reference is made to the pincerlike manipulation instrument of U.S. Pat.No. 5,314,477.

Because of the strong forces, it is necessary to provide a certainstructural height for the engagement elements; for instance, thereceiving bores must have a certain diameter. This dictates a minimumstructural height for the upper part and for the lower part, and inconventional intervertebral implants, the structural heights of theupper part and lower part are thus added together, so that even if theupper and lower parts rest directly on one another, a relatively greatstructural height of the intervertebral implant is still unavoidable.

SUMMARY OF THE INVENTION

It is the object of the invention to embody an intervertebral implant ofthis generic type in such a way that the minimum structural height isreduced, to make it easier to insert the intervertebral implant into theintervertebral space.

In an intervertebral implant of the type described at the outset, thisobject is attained in accordance with the invention in that it isproposed that the upper part and lower part each have protrusions andrecesses aimed at the respectively other part, which are offsetlaterally from one another in such a way that when the upper part hasbeen brought close to the lower part they mesh with one another; andthat the engagement elements on the upper part and on the lower part areeach disposed in protrusions of these parts in such a way that theengagement elements of the upper part and lower part are located side byside and at least partly overlap in the direction of the height of theintervertebral implant.

In such an embodiment, a minimal structural height of the twointervertebral implant parts resting on one another can be attained,since the engagement elements, which cannot fall below a minimalstructural height, are each disposed in protrusions of the upper partand lower part, or in other words in the parts of the upper part andlower part that have the greatest structural height. These regions ofgreat structural height are embodied as protrusions, next to which arerespective recesses, into which the protrusions of the respectivelyother part can dip. As a result, on the one hand, the engagementelements for the manipulation instruments are located side by side, andon the other, they can at least partly overlap, so that the totalstructural height of the parts resting on one another of theintervertebral implant can be reduced markedly compared to conventionalintervertebral implants. The result is accordingly an internestedarrangement of the upper and lower parts, with maximal exploitation ofthe available material height.

It is favorable if the engagement elements are insertion openings forpinlike retaining elements of a manipulation instrument; because of thedescribed construction, these insertion openings can have a relativelylarge diameter and can thus receive strong retaining pins, andnevertheless a relatively low structural height of the intervertebralimplant with parts resting directly on one another is obtained.

It is advantageous if the insertion openings extend substantiallyparallel to the support faces; once again, this prevents an increase inthe structural height of the intervertebral implant parts.

In a preferred embodiment, it is provided that the lower part has acentral indentation, opposite the lower support face, which indentationis surrounded by a U-shaped edge. Thus with the lower part and upperpart resting directly on one another, the indentation serves to receivea protrusion on the upper part.

It is advantageous if the upper part has a central protrusion that fitssubstantially in complimentary fashion into the indentation; that is,the total volume of the indentation is utilized for the protrusion.

It is also advantageous if the engagement elements of the lower part aredisposed on the two ends of the U-shaped edge, or in other words arelocated on the outside.

Conversely, the engagement elements of the upper part can be disposed onthe central protrusion of the upper part, or in other words are locatedfarther inward than the engagement elements of the upper part.

In particular, the engagement elements of the upper part can be disposednear the lateral edges of the central protrusion, so that for the upperpart as well, the spacing of the engagement elements can be selected tobe relatively great; as a result, both the upper part and the lower partcan be reliably secured against skewing.

It should already be noted here that the words “lower part” and “upperpart” do not necessarily say anything about the installed position ofthe intervertebral implant in the spinal column; the part called the“lower part” could in fact be above in the spinal column. Therefore,these parts may also be referred to as first and second parts havingouter and inner surfaces. What is essential is merely that the upperpart and lower part define the intervertebral implant on opposite sidesof the implant.

It is especially advantageous if the upper part and/or the lower part isembodied in substantially platelike fashion; these parts naturally, inaccordance with the design of the invention, have protrusions andrecesses that are oriented toward the respectively other part. Theplatelike embodiment, however, leads as a whole to a very low structuralheight of the intervertebral implant.

In a preferred embodiment, the lower part and the upper part each have arespective receptacle for a pivot insert. This pivot insert, which isplaced between the upper part and lower part after the insertion of theintervertebral implant, supports the upper part and lower part againstone another; it takes on a resilient function, for instance, andfurthermore leads to a certain pivotability of the two parts of anintervertebral implant relative to one another, so that a pivotabilityof the adjacent vertebra is thus attainable as well.

In particular, it is advantageous if the pivot insert has at least onespherical support face, which engages the correspondingly sphericallyshaped receptacle.

It is favorable if the spherical receptacle is disposed in the centralprotrusion of the upper part.

It is also advantageous if the central indentation of the lower partforms the receptacle for the pivot insert.

According to a preferred embodiment of the invention, it is providedthat the pivot insert can be inserted from the side into the receptacle,which has the engagement elements for a manipulation instrument. This isthe side from which the upper part and lower part are introduced intothe intervertebral space, and it is also from this side that the pivotinsert can then be thrust between the already-inserted parts of theintervertebral implant.

It is favorable if the pivot insert is insertable into the receptaclealong a guide.

In that the insert as well is preferably embodied substantially inplatelike fashion.

An especially favorable design is obtained if the insert substantiallycompletely fills up the central receptacle and with its sphericalsupport face protrudes from the receptacle.

BRIEF DESCRIPTION OF THE DRAWINGS

The ensuing description of preferred embodiments of the invention servesin conjunction with the drawing to provide further explanation. Shownare:

FIG. 1: a perspective exploded view of an intervertebral implant with anupper part, a lower part, and a pivot insert that can be insertedbetween them;

FIG. 2: a perspective exploded view of the upper part and the lower partof the intervertebral implant, without an inserted pivot insert;

FIG. 3: a view similar to FIG. 2 with the pivot insert inserted into thelower part;

FIG. 4: a perspective view of the upper part and the lower part of theintervertebral implant with maximum mutual proximity;

FIG. 5: a front view of the intervertebral implant of FIG. 4;

FIG. 6: a perspective view of the intervertebral implant with the pivotinsert inserted; and

FIG. 7: a cross-sectional view of the intervertebral implant of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The intervertebral implant 1 shown in the drawing includes three parts,namely a platelike upper part 2, a platelike lower part 3, and asubstantially platelike pivot insert 4.

The upper part 2 is embodied flat on its top, thus creating a supportface 5, on which various kinds of protrusions 6, 7 are disposed whichserve the purpose of anchoring the upper part 2 in a vertebra thatrests, with its end face toward an intervertebral space, on the supportface.

The upper part 2 is substantially rectangular in cross section; in theexemplary embodiment shown, a longitudinal edge 8 curves outward.

On the two short sides of this rectangle, the thickness of the platelikeupper part 2 is less than in the central region, so that along the shortsides of the upper part 2, downward-pointing recesses 9 each extendingparallel to these edges are formed that are open toward the outside. Thecentral region of the upper part 2 is located between the two recesses 9and thus has a greater thickness or height and thus forms adownward-pointing protrusion 10 embodied between the two recesses 9.This protrusion is defined by an underside 11, which extendssubstantially parallel to the support face 5 and in which there is aspherical indentation 12, which forms a bearing plate for the pivotinsert 4.

The lower part 3 of the intervertebral implant 1 is also platelike inembodiment and on its underside has a flat support face 13 withprotrusions 14 and 15, which correspond to the protrusions 6 and 7 ofthe support face 5. On the side remote from the support face 13, thethickness of the lower part 3 is less in the central region than in anouter region. This outer region of greater thickness has the form of aU, with two parallel legs 16, 17, which extend parallel to the shortedges of the lower part 3, which in cross section is embodied similarlyto the upper part 2, and with a crosspiece 18 that connects the two legs16 and 17 on one end. The region enclosed by the legs 16 and 17 and thecrosspiece 18 forms a central indentation 19, whose area issubstantially equivalent to the area of the central protrusion 10 of theupper part 2, while the disposition and length of the legs 16 and 17correspond essentially to the disposition and length of the recesses 9on the upper part 2. As a result, it is possible to place the upper 2and lower part 3 on one another in such a way that the centralprotrusion 10 of the upper 2 dips into the central indentation 19, whilethe legs 16 and 17 of the lower part 3 dip into the recesses 9 of theupper part 2 (FIG. 4); in this position, the upper part 2 and lower part3 have maximum proximity to one another and a minimal structural height.

The dimensions are selected such that the various recesses areessentially filled completely by the protrusions dipping into them.

Blind bores 20 and 21 are machined into the two legs 16 and 17 of thelower part 3, extending parallel to these legs 16, 17 from their freeends; the diameter of these bores is relatively great in proportion tothe height of the legs 16, 17, and this diameter is in fact greater thanthe thickness or height of the lower part 3 in the region of the centralindentation 19.

Blind bores 22 and 23, which extend parallel to the blind bores 20 and21 in the lower part 3, are machined into the central protrusion 10 ofthe upper part 2, in the vicinity of its side edges. These blind bores22 and 23 again have a relatively great diameter, which corresponds to asubstantial portion of the height of the protrusion 10 and is greaterthan the thickness of the upper part 2 in the region of the recesses 9.

When the upper part 2 and lower part 3 rest tightly against one anotherin the manner described, the blind bores 20 and 21 of the lower part 3and the blind bores 22 and 23 of the upper part 2 overlap at leastpartly in the direction of the height of the intervertebral implant 1,as is clearly shown in FIGS. 4 and 5.

The blind bores 20, 21, 22 and 23 serve as receptacles for pinlikeextensions of a manipulation instrument, not shown in the drawing, andthus form engagement elements for this manipulation instrument, which inthis way separately engages the upper part 2 and the lower part 3. Withthis manipulation instrument, it is possible to introduce the upper part2 and the lower part 3 of the intervertebral implant 1 into anintervertebral space; the very low structural height of theintervertebral implant 1 facilitates this introduction, which can bedone essentially without major widening of the intervertebral space.

After the introduction of the upper part 2 and lower part 3 in this way,the two parts of the intervertebral implant 1 can be spread apart; thatis, their spacing is increased, for instance with the aid of themanipulation instrument that is holding the upper 2 and the lower part3.

In this spread-open position of the upper part 2 and lower part 3, it ispossible to thrust the pivot insert 4 between the upper part 2 and thelower part 3.

This pivot insert is constructed essentially in the shape of a plate,which has a flat underside 24 and a spherically upward-curved top side25. The outer dimensions of the platelike pivot insert correspond tothose of the central indentation 19 in the lower part 3, so that thepivot insert 4 can be thrust into this indentation, filling it up,specifically from the side toward which the blind bores 20, 21, 22, 23open. Guide strips 26 on the side edges of the pivot insert 4 engagecorresponding guide grooves 27 in the legs 16, 17, so that an insertionguide for the pivot insert 4 is formed that fixes it in the lower part 3after its insertion. The inserted pivot insert 4, after insertion, fillsup the indentation 19 and protrudes with its spherically curved top side25 upward past the top side of the lower part 3; the spherical top side25 dips in complimentary fashion into the spherically curved indentation12 on the underside of the protrusion 10, where with the upper part 2 itforms a ball joint, which enables a certain pivotability of the upperpart 2 relative to the lower part 3 (FIG. 7).

The pivot insert 4 can have a detent protrusion 28 on its flat underside24; when the pivot insert 4 is inserted into the lower part 3, thisprotrusion locks elastically into a detent recess 29 that is located onthe bottom of the indentation 19; as a result, the pivot insert 4 isalso fixed in the insertion direction in the indentation 19.

The upper part 2 and lower part 3 are preferably made of physiologicallysafe metal, such as titanium, while the pivot insert 4 preferablycomprises a likewise physiologically safe plastic material, such aspolyethylene. These support faces 5 and 13 can be embodied in anespecially bone-compatible way; for instance, this surface can beroughened by a coating, so that optimal anchoring to the adjacent bonematerial is obtained.

The invention may also be described as follows, which description is thefull equivalent of the preceding discussion. An upper part 2 has anupper surface 5 for engaging a vertebrae and a lower surface whichcomprises a downward pointing protrusion 10 between side recesses 9 anda rounded portion, preferably in the form of a concave sphericalindentation 12. A lower part 3 has a lower surface 13 for engaging avertebrae. A pivot insert 4, when joined to the lower part 3, as shownfor example in FIG. 3, provides a convex upper surface portion 25,preferably spherical, in operational engagement with the rounded portion12 of the upper part.

The lower part 3 and pivot insert 4 may, taken together, be described asa lower part formed in two pieces, namely the elements 3 and 4, whereinthe element 3 may be referred to as a lower piece and the element 4 maybe referred to as an upper piece.

The upper and lower parts include on their upper surface and lowersurface, respectively, protrusions 7 and 14 which may also be referredto as anchors, which anchor the upper and lower parts, respectively,into the adjacent vertebrae that form the intervertebral space and restagainst the respective upper and lower surfaces.

As shown in the figures, the anchors 6 and 14 each have a zigzag edgewhich comprise teeth. As best shown in FIG. 7, anchor 6 is greater inheight than the remainder of the upper part 2, i.e., from surface 5 tothe bottom of protrusion 10. Similarly, anchor 14 is greater in heightthan the remainder of the lower part 3, i.e., from lower surface 13 tothe top of walls 16, 17 and 18. As also shown in the figures, in thepreferred embodiment, the length of the anchors 6 and 14, i.e., in thedirection from the anterior to the posterior thereof, is greater thanone half of the overall dimension of its respective part from itsanterior to its posterior, passing through that anchor.

The lower part comprises three walls including parallel side walls 16and 17 and a rear wall 18. These walls form between them a centralindentation 19 which comprises a recess with a generally flat surface.The fourth side of the recess is open. The pivot insert 4 has a detent28 that snap-fits into a detent recess 29 formed in the generally flatsurface of recess 19.

As best shown in FIGS. 2, 4 and 5, in the absence of pivot insert 4, theprotrusion 10 of upper part 2 can fit down between walls 16, 17 and 18of the lower part 2. This fitting of protrusion 10 within the recess 19,surrounded by walls 16, 17 and 18 may be referred to as “nesting” sincethe protrusion 10 essentially “nests” within recess 19. With the upperand lower parts in this nested condition, as shown in FIGS. 4 and 5, thecombined height of the upper and lower parts 2 and 3, i.e., the heightfrom surface 13 to surface 5, is less than the total additive height ofthe upper and lower parts, taken separately, i.e., less than the totalof the height from surface 13 to the top of walls 16, 17 and 18 plus theheight from surface 5 to the bottom of protrusion 10.

To reach its final destination within an intervertebral space, theimplant must of course be moved along a path from outside of thepatient, into the patient, and then into the intervertebral space. Inthe illustrated embodiment, as described above, instruments would engageapertures 20, 21, 22 and 23 to move the implant along a path. Theanchors 6 and 14 are parallel to this path. As a point of reference,lateral planes parallel to the direction of this path pass throughopposed side surfaces of the parts. Thus, in the illustrated embodiment,the path would be parallel to the front to rear (anterior to posterior)direction, wherein, during insertion, the rear (posterior) of theimplant would constitute the lead end and the front (anterior) thereofwould constitute the trailing end.

Although the invention has been described in detail with respect topreferred embodiments thereof, it will be apparent that the invention iscapable of numerous modifications and variations, apparent to thoseskilled in the art, without departing from the spirit and scope of theinvention.

1. (canceled)
 2. An intervertebral implant sized and configured toreplace a natural disk removed from an intervertebral space betweenopposing end faces of a first vertebra and an adjacent second vertebra,the intervertebral implant having a height extending in a firstdirection, a width extending in a second direction, and a depthextending in a third direction, where the first, second, and thirddirections are each perpendicular to each other, the implant having amaximum width greater than an implant maximum depth, the implantcomprising: a first part having an outer surface facing substantially inthe first direction and configured to rest against the end face of thefirst vertebrae and having an inner surface facing substantiallyopposite the outer surface, the inner surface comprising a spherical,concave portion; a second part having an outer surface facingsubstantially opposite the outer surface of the first part andconfigured to rest against the end face of the second vertebrae and anopposing inner surface facing substantially in the first direction; aremovable pivot insert located between the first part and the secondpart along the first direction and having an upper surface facingsubstantially in the first direction, a lower surface that facesopposite the upper surface of the insert, and a sidewall that extendscircumferentially around the pivot insert, the sidewall connecting theupper and lower surfaces of the pivot insert, the pivot insert includinga spherical, convex portion on the upper surface configured tooperatively engage the spherical, concave portion of the first partwhere the spherical, convex portion is smaller in area, transversely,than the upper surface of the insert; and a first anchor located on theouter surface of the first part, the first anchor having a heightextending in the first direction, a width extending in the seconddirection, and a depth extending in the third direction, wherein thefirst anchor has a first anchor base connecting the first anchor to theouter surface of the first part and wherein the first anchor base has afirst base depth that extends depthwise substantially along a firstmidline that extends in the third direction and that is locatedessentially midway across the width of the outer surface of the firstpart, and a first base width that extends in the second direction and isless than the first base depth, where the first anchor has a firstcenter height measured from the outer surface of the first part at afirst center position located at approximately a center of the firstmidline and the first anchor has a mid-anchor width measured at alocation half of the first center height and wherein the first centerheight is greater than the mid-anchor width of the first anchor, and thefirst base depth is greater than the mid-anchor width of the firstanchor, and wherein the first anchor has a first anchor top opposite thefirst anchor base having a first top width less than the first basewidth; a second anchor located on the outer surface of the second part,the second anchor having a height extending in the first direction, awidth extending in the second direction, and a depth extending in thethird direction, wherein the second anchor has a second anchor baseconnecting the second anchor to the outer surface of the second part andwherein the second anchor base has a second base depth that extendsdepthwise substantially along a second midline that extends in the thirddirection and that is located essentially midway across the width of theouter surface of the second part, and a second base width that extendsin the second direction and is less than the second base depth, wherethe second anchor has a second center height measured from the outersurface of the second part at a second center position located atapproximately a center of the second midline and the second anchor has amid-anchor width measured at a location half of the second center heightand wherein the second center height is greater than the mid-anchorwidth of the second anchor, and the second base depth is greater thanthe mid-anchor width of the second anchor, and wherein the second anchorhas a second anchor top opposite the second anchor base having a secondtop width less than the second base width; wherein the first centerheight is greater than a height of a remainder of the first partmeasured from the outer surface to the inner surface of the first partat the first center position; and wherein the first midline is the onlylocation on the outer surface of the first part having an anchor havinga maximum height equating to the first center height; and wherein thesecond midline is the only location on the outer surface of the secondpart having an anchor having a maximum height equating to the secondcenter height.
 3. The intervertebral implant of claim 2 wherein thesecond part comprises a recess extending inwardly from the inner surfaceof the second part toward the outer surface of the second part, and thepivot insert comprises a protrusion extending from the lower surface ofthe pivot insert, and wherein the protrusion extends into the recess ofthe second part.
 4. The intervertebral implant of claim 3 wherein theouter surfaces of the first and second parts comprise a roughenedcoating.
 5. The intervertebral implant of claim 4 wherein the uppersurface of the pivot insert further comprises a substantially planarportion facing substantially in the first direction and bounded by thesidewall and the spherical convex portion of the insert.
 6. Theintervertebral implant of claim 5 wherein the protrusion of the pivotinsert has a width in the second direction and a height in the firstdirection where the width is greater than the height of the protrusionand the protrusion has an outer surface that is curved when viewed in aplane extending in the first and third directions.
 7. An assemblage ofrelated components comprising a first part, a second part, and a pivotinsert that are each sized to form, when assembled, an intervertebralimplant sized and configured to replace a natural disk removed from anintervertebral space between opposing end faces of a first vertebra andan adjacent second vertebra, the intervertebral implant having a heightextending in a first direction, a width extending in a second direction,and a depth extending in a third direction, where the first, second, andthird directions are each perpendicular to each other, the implanthaving a maximum width greater than an implant maximum depth, theassemblage comprising: the first part including an outer surface facingsubstantially in the first direction and configured to rest against theend face of the first vertebrae and having an inner surface facingsubstantially opposite the outer surface, the inner surface comprising aspherical, concave portion; the second part including an outer surfacefacing substantially opposite the outer surface of the first part andconfigured to rest against the end face of the second vertebrae and anopposing inner surface facing substantially in the first direction; thepivot insert configured to be located between the first part and thesecond part and having an upper surface facing substantially in thefirst direction, a lower surface that faces opposite the upper surfaceof the pivot insert, and a sidewall that extends circumferentiallyaround the pivot insert connecting the upper and lower surfaces of thepivot insert, the pivot insert including a spherical, convex portion onthe upper surface configured to operatively engage the concave portionof the first part where the spherical, convex portion is smaller inarea, transversely, than the upper surface of the pivot insert; andwherein the first part further comprises a first anchor located on theouter surface of the first part and having a height extending in thefirst direction, a width extending in the second direction, and a depthextending in the third direction, wherein the first anchor has a firstbase connecting the first anchor to the outer surface of the first partand wherein the first base has a first base depth that extends depthwisesubstantially along a first midline that extends in the third directionand that is located essentially midway across the width of the outersurface of the first part, and a first base width that extends in thesecond direction and is less than the first base depth, where the firstanchor has a first center height measured from the outer surface of thefirst part at a first center position located at approximately a centerof the first midline and the first anchor has a mid-anchor widthmeasured at a location half of the first center height and wherein thefirst center height is greater than the mid-anchor width of the firstanchor, and the first base depth is greater than the mid-anchor width ofthe first anchor, and wherein the first anchor has a first anchor topopposite the first anchor base having a first top width less than thefirst base width; wherein the second part further comprises a secondanchor located on the outer surface of the second part and having aheight extending in the first direction, a width extending in the seconddirection, and a depth extending in the third direction, wherein thesecond anchor has a second base connecting the second anchor to theouter surface of the second part and wherein the second base has asecond base depth that extends depthwise substantially along a secondmidline that extends in the third direction and that is locatedessentially midway across the width of the outer surface of the secondpart, and a second base width that extends in the second direction andis less than the second base depth, where the second anchor has a secondcenter height measured from the outer surface of the second part at asecond center position located at approximately a center of the secondmidline and the second anchor has a mid-anchor width measured at alocation half of the second center height and wherein the second centerheight is greater than the mid-anchor width of the second anchor, andthe second base depth is greater than the mid-anchor width of the secondanchor, and wherein the second anchor has a second anchor top oppositethe second anchor base having a second top width less than the secondbase width; wherein the first center height is greater than a height ofa remainder of the first part measured from the outer surface of thefirst part to the inner surface of the first part at the first centerposition; wherein the first midline is the only location on the outersurface of the first part having an anchor having a height equating tothe first center height, and wherein the second midline is the onlylocation on the outer surface of the second part having an anchor havinga height equating to the second center height.
 8. The intervertebralimplant of claim 7 wherein the second part comprises a recess extendinginwardly from the inner surface of the second part toward the outersurface of the second part, and the pivot insert comprises a protrusionextending from the lower surface of the pivot insert, and wherein theprotrusion extends into the recess of the second part.
 9. Theintervertebral implant of claim 8 wherein the outer surfaces of thefirst and second parts comprise a roughened coating.
 10. Theintervertebral implant of claim 9 wherein the upper surface of the pivotinsert further comprises a substantially planar portion facingsubstantially in the first direction and bounded by the sidewall and thespherical convex portion of the insert.
 11. The intervertebral implantof claim 10 wherein the protrusion of the pivot insert has a width inthe second direction and a height in the first direction where the widthis greater than the height of the protrusion and the protrusion has anouter surface that is curved when viewed in a plane extending in thefirst and third directions.